Firstly, it should be recalled that in the particular case of helicopters, the system designed to absorb energy generally includes provision to limit force peaks in the event of the helicopter falling at high speed due to a breakdown of its support system. When the landing gear of the helicopter is of the rocker type, the shock absorber for normal use is hinged at its bottom end to the rocker arm and at its top end to the structure of the helicopter, and it extends substantially vertically, such that the reaction from the ground via the associated wheel or wheels tends to pivot the rocker arm and thrust the rod of the shock absorber into the body thereof. As a result, in the event of a crash landing, since the vertical impact speed is much greater than in normal operation, it is necessary to provide force peak-limiting means capable of coming into action as quickly as possible.
Thus, shock absorbers are known having a low-pressure chamber and fitted with force peak-limiting means in the form of a separator piston delimiting a chamber containing gas under high-pressure. In that case, under conditions of normal use, the vertical reaction from the ground causes the rod to be retracted into the main body, thereby compressing a volume of gas contained in the low-pressure chamber, with said retraction movements being braked by a throttling device. In contrast, in the event of a crash landing, since the impact speed is much higher, the pressure induced by the throttling device is communicated by the oil in the separator piston cylinder, and since the pressure then applied to the separator piston exceeds the pressure to which the high-pressure chamber is inflated, said piston moves downwards, compressing said chamber.
Such shock absorbers are used at present for fitting to the landing gear of airplanes or of helicopters, however they are not designed also to provide an actuator function for lowering and raising drive purposes, such that said actuator function is provided by a separate driving actuator.
In some cases, there is a requirement for a shock absorber that is also capable of providing an actuator function, which we refer to as a "shock-absorbing actuator".
A shock-absorbing actuator is already known that comprises a cylindrical body, and a hollow rod having an open end forming an annular piston that slides in sealed manner inside the cylindrical body, together with hydraulic shock-absorbing means including a diaphragm and surmounted by a volume of hydraulic fluid that delimits a chamber containing gas (e.g. nitrogen) at low-pressure, and also force peak-limiting means that act in the event of a crash landing.
Document FR-A-2 608 242 describes a shock-absorbing actuator of that type, and more precisely it comprises a main body disposed below a rod-piston, the bottom end of the main body including a high-pressure chamber which is adjacent to a low-pressure chamber via a flexible separation membrane capable of bearing against a perforated rigid support secured to said body, together with a throttling device that brakes the displacements of the rod inside the main body by throttling the fluid contained in said body and adjacent to the low-pressure chamber. The rod-piston is hollow, and it receives a separator piston that delimits two hydraulic chambers. The throttling device includes a central pressure-release valve that acts in the event of the rod being forced in suddenly, and the device is also associated with a cylindrico-conical tube that absorbs energy by undergoing plastic deformation.
The shock-absorbing actuator described in the above-specified document provides the actuator function by connecting the two hydraulic chambers to a hydraulic distributor valve block. The chambers comprise a chamber for retracting or raising the landing gear by causing the rod to be retracted into the body, and a landing gear lowering or extending chamber that causes the rod to extend out from the body, and the hydraulic distributor valve block is capable of feeding hydraulic fluid under pressure to said chambers or of allowing them to empty into the tank of the aircraft's hydraulic circuit.
Such a shock-absorbing actuator nevertheless remains relatively complicated, and in particular it requires a throttling device that is servo-controlled in velocity, and which needs to be adjusted accurately.